Please note that while this guide covers the best ways to power-level your lockpicking, it is perfectly feasible to level the vast majority of your skill simply by choosing to work quests that involve humanoids that can be pickpocketed for lockboxes, which can then be picked to raise your skill, and which also will sometimes contain Rogue poisons, thus increasing the value of this approach.

Early on you might get some Inv box 01 [Practice Locks] from an engineer to increase your lockpicking up to skill level 80. The quest to obtain Lockpicking will point you towards plenty of practice chests.

The best way for Alliance rogues to get their lockpicking skill up at level 16-20 (after getting the Pick Lock skill) is Alther's Mill in the Redridge Mountains region. The quest [20] Alther's Mill requires lockpicking a chest in Alther's Mill for a quest item and is close to the practice lockboxes. These chests respawn very quickly (you can stand between a couple or three and alternate between them) and may be practiced upon from 1 to over 100 skill. You do not have to be on the quest to pick these, so you can come back later to raise your skill when your level based skill cap is higher. If you do the quest at level 16, you should probably return at least once when you are over level 20. Horde rogues can similarly gain early levels from an equivalent quest on the ship off the Merchant Coast, south of Ratchet in the Barrens. (Note: The above mentioned chest are not faction specific. If preferred Alliance characters could just as easily skillup lockpicking on chests found in the ship at the Merchant Coast. Vise-versa, it is possible for Horde characters to level on the chests found in Alther's Mill.)

1-70 - The Barrens, Ship off of merchants coast. (Has yellow spikes in the front you can jump on to board)
Respawn timer 0.5 seconds. 10 Box spawn points.

The rogue poisons quests for Horde and Alliance provide the next opportunity for solid skill gains. The quest can be picked up at level 20, although it may be too difficult for many players at level 20. The item required to complete this quest is contained inside a locked chest that can be picked repeatedly up to 170. (You are likely to hit your skill level cap first, if you are on the poison quest.) If you've already finished the quest, you can still use the chests to gain skill. The chests don't highlight when you mouse over them after the first time (when you obtain the quest item), so you have to activate your lockpicking skill first and then click the chest. The chest will be empty if you already have the quest item, or unpickable if you have completed the quest. Be sure to close the empty loot window in order to speed along the chest respawning (it is very quick if the loot window is not left open.) The poison Touch of Zanzil is applied every time the chest is lockpicked. The poisons can be dispelled if you have the cure from the poison quest chain, Anti-Venom skill (and Anti-Venom in your pack) from the First Aid profession, or by the dwarf racial Stoneform. It has also been confirmed that Jungle Remedy also cures this poison but requires the user to be level 22 or higher. The boss guarding the chest will probably respawn a couple of times while you are leveling your lockpicking skill on this chest. (It appears that either faction could use the opposite faction chest; not tested.)

Level 1 Battered Footlockers can be pickpocketed from the mobs around Durnholde Keep in the Hillsbrad Foothills. Note again that these are level 1, so if you have already leveled up using the lockbox in the rogue poisons quest, these will be grey and therefore useless for leveling your skill. Inside Durnholde Keep there are footlockers scattered around the grounds that require a lockpicking skill of 100 to open which can help you level your skill.

At Bluegill Marsh in the Wetlands, there are Battered Footlockers with varying skill level requirement of 90, 100, and 110.

If you are level 30-40 you can go to Desolace, bring 2–3 hours worth of water breathing potions, and pick your way into the mid 200s. There are Waterlogged Footlockers around (37,21) off the northwest shore. There are level 33 mobs you will have to deal with while doing this. There are also Mossy Footlockers in the deeper areas that can get you to 225. These can be found in a NE running trench near the big island off the coast, around (36,11). Though they are few and far between here. A better place to find these is in the lake that surrounds the Sunken Temple in the Swamp of Sorrows.
Alternate Path.

If you don't like the water, you can head to Angor Fortress in the Badlands. There are two types of footlockers inside the fortress, Battered Footlockers and Dented Footlockers. The Battered Footlockers are located upstairs, and you will be able to open them immediately at 150; however, the Dented Footlockers will require a skill of 175 to open.

At 225, go to Searing Gorge. Down in the Slag Pit are [Dented Footlockers]. You can open all of them, and again pickpocket for junkboxes. Additionally, you can travel to the Swamp of Sorrows. A water-breathing potion makes circling the deep water around the Sunken Temple much easier, as [Mossy Footlockers] spawn throughout the area. Stay in either of these locations until 250-255.

The best place to raise your skill from 250 to 300 is in Blackrock Depths. Simply go in, turn to the left and unlock the Shadowforge Gate. Continue forward and take the first right, picking the next Shadowforge Gate. Pick the first door on the left, the East Garrison Door, and then head through the door, staying on the bottom level. If you are level 60 or above, pick the large wooden object sticking out of the ground below the giant gear, The Shadowforge Lock; lower levels can aggro the mobs around it. Pickpocket the 10 humanoid mobs in the room and you should average either 1 Sturdy Junkbox or 1 Heavy Junkbox each run. You can also lockpick cell doors if you are able to solo small groups before them. Then head out of the instance and reset it before doing all four again. Note that you can only reset the instance 4 times before it disables you from entering, saying "You have entered too many instances recently". This will yield 5 runs. This is valid for the whole account and resets roughly an hour after you entered the first time. Keep in mind that a number of mobs will be able to see through stealth, regardless of your level, so be prepared.

Another place for 250-300 is Tyr's Hand in Eastern Plaguelands. Pickpocket the mobs and find all of the [Scarlet Footlockers]. In the time it takes to take a full circle of Tyr's Hand, the starting point will be pickpocketable and have boxes respawned. Do this until 300.

Another spot at Searing Gorge is the Stonewrought Pass. You can pick the doors on either side at around 225, however the Loch Modan side has an Alliance guard so Horde may find it easier to simply level on the Searing Gorge side. The door quickly closes after being opened, so you can continuously pick the door until it is grey.